Does the Principled shader alleviate the need for custom PBR setups?
Yes - and no.

For most 'real world' materials, the Principled shader should be able to produce very good results. However, it is not perfect and cannot deal with every situation - otherwise it would have far more parameters, inputs and sliders than would be practical.

The existing principled shader should be fine for pretty much all 'simple' materials - those where the visible effect is due to the material and surface properties alone - or where there is a single layer of 'clearcoat' that is within the capabilities of the 'clearcoat' parameters.

This should be sufficient for, say, 99 percent of materials (or more!). However, there are feasible situations where the principled shader is insufficient - and under those situations you would again need to combine multiple shaders. For example, a surface with multiple 'clearcoat' layers  with different properties of each layer, or a surface where the micro-detail has a significant effect on the appearance (such as the fine structure of butterfly wing scales or the irridescence of beetle wingcases).